A membrane can be defined as a barrier separating two fluids, which barrier prevents hydrodynamic flow therethrough, so that transport between the fluids is by sorption and diffusion. The driving force for transport through the membrane is pressure, concentration or a combination of both. During operation permeate molecules dissolve into such a membrane at its upstream surface followed by molecular diffusion down its concentration gradient to the downstream face of the membrane. At the downstream face of the membrane the permeate is evaporated or dissolved into the adjacent fluid phase. The property of the membrane describing the rate of transport is called is permeability.
The importance of membranes in chemical technology for separating liquid and/or gaseous components from one another is rapidly growing, since the membrane permeation process is particularly useful as a separation technique whenever conventional separation methods cannot be used economically to get reasonable separation. Separation by means of membranes has a further advantage in that the components to be separated are not subjected to thermal loads and not changed in chemical structure.
Membranes can be distinguished as to their microstructural forms in porous ones and non-porous or dense ones. Membranes are usually nominated as porous when they contain voids that are large in comparison with molecular dimensions. Transport of permeates occurs within the pores of such membranes. Porous membranes have high transport rates which, however, is accompanied with a very poor selectivity for small molecules, and are therefore less suitable for gas separation techniques.
Dense membranes have on the contrary the ability to transport species selectively and are therefore applicable for molecular separation processes, such as gas purification. With such dense membranes, even molecules of exactly the sme size can be separated when their solubilities and/or diffusivities in the membrane differ significantly. A problem with dense membranes is the normally very slow transport rates. To attain acceptable transport rates, required for commercial application in separation processes where productivity is of paramount concern, it is necessary to make such membranes ultrathin. This can be construed from the following equation applicable for gas separation ##EQU1## wherein N represents the permeation rate, P is the permeability i.e. product of solubility and diffusivity, (p.sub.1 -p.sub.2) is the pressure difference over the membrane, and L is the membrane thickness. Similar equations are known for solid/liquid, liquid/liquid and gas/liquid separation by means of dense membranes.
From the above it will be clear that the amount of permeation per unit surface for a given material of the membrane and a given permeate depends upon the thickness of the membrane.
Various techniques are known for producing very thin membranes. The most common methods are melt extrusion, calendering and solvent casting. Melt extrusion should be carried out with rather complex equipment and it sets requirements, among others thermal stability, to the material to be extruded. Calendering does not permit the production of membranes with a thickness less than about 50 .mu.m. The most preferred production method is solvent casting, which involves forming a solution of the membrane material, normally consisting of a polymer, and casting it onto a liquid substrate to produce a thin liquid layer which is then dried to form the solid membrane film. Essential in this method is that the solution has the ability to spread spontaneously onto the liquid substrate. In U.S. Pat. No. 4,192,842 a method for producing membranes is disclosed, wherein a solution of a methylpentene polymer in a solvent is spread out over a water surface. According to the disclosure in U.S. Pat. No. 4,192,842 the spreadability is enhanced by adding an organopolysiloxane-polycarbonate copolymer to the solution. It has been found that merely applying a solvent of a methylpentene polymer did not result in a dense, hole-free membrane, meeting the required selectivity for proper operation of the membrane.
Further examples of membrane production by solvent casting are given in European patent publication No. 31725. As described in the specification pertaining to this publication, an organic compound with a distribution coefficient of from 0.5 to 35 is preferably added to the polymer solution. The distribution coefficient is the ratio of the concentration of the organic compound in the polymer solution to that in water, forming the liquid substrate. According to said latter publication, the organic compounds may be alicyclic or aromatic alcohols, ketones, amines, aldehydes, carboxylic acids, peroxides and mixtures of these. Once the polymer solution with the above additional organic compound has been spread over a liquid support, it is believed according to said European patent publication that most of the additional organic compound is removed from the membrane forming solution by being dissolved in the liquid support. Although in first instance the organic compound may be a useful help in wide spreading of the polymer solution over the surface of a liquid support, the effect of the organic compound drastically diminishes due to its escape into the liquid support. This behavior of the organic compound means that during the drying of the polymer solution the surface tension of the liquid support is reduced, resulting in instability of the membrane and possible generation of holes in the membrane, especially when the membrane solidification process is rather slow and/or proceeds after the desolvation of the membrane.